NORTH Somerset MP Liam Fox has called on Britain to quit the EU unless it can change its relationship with Brussels.

The former Defence Secretary yesterday became the most senior Tory MP to raise the prospect of withdrawing from the European Union, ramping up the pressure on Prime Minister David Cameron on the issue.

In his first speech on Europe since quitting the Cabinet in a lobbying scandal, Dr Fox said: "I do not believe that Britain's national interest is served by its current relationship with the EU."

Pressure has been mounting on Mr Cameron from within his own party to hold a referendum on Britain's membership of the union.

The Prime Minister has hinted he would be open to the idea but has insisted now is not the time.

Dr Fox agreed with him, saying: "I too believe that a referendum will be vital but I believe that having one now would be a huge error, with enormous tactical risks."

Instead, Dr Fox said Britain should negotiate a new relationship with the EU, which was based on economics rather than politics, which should be put to a referendum.

He said: "If, on the other hand, this approach is rejected outright or falls short of necessary 'red lines', then we would have no alternative but to recommend rejection and consider departure from the EU."

Dr Fox said he trusted Mr Cameron to tackle the issues and act in the "best interests" of the British people.

But he admitted that making changes while the Tories were in coalition with the Europhile Lib Dems would not be easy.

Dr Fox denied that his speech was part of a comeback effort after his resignation last year, and said it would not have been possible for him to make such comments from the front bench.

He added: "I don't imagine I would be giving this speech from the heart of government. At least, I would not be at the heart of government for more than about 25 minutes."

Stephen Williams, the Liberal Democrat MP for Bristol West, said: "What I suspect Liam is doing is positioning himself within the Conservative Party".

Mr Williams also said the Prime Minister was trying to avoid losing right-wing votes to the UK Independence Party, which is leading calls for an 'in/out' referendum.

He said: "I think there is a lot of domestic politics at play here."

Mr Williams said he would be happy to "argue passionately" in favour of the EU if a referendum was held but said to do so in the middle of the Eurozone crisis would be "massively destabilising".

His fellow Lib Dem, Thornbury and Yate MP Steve Webb, agreed.

The pensions minister said: "All our efforts should be geared towards trying to protect British jobs. There is bound to be a lively debate on Europe coming – pressure is growing – but not now."

Mr Cameron said yesterday that the UK faced "big choices" about its relationship with the EU but the short-term priority was to deal with "instability and chaos" in Europe.

He told MPs: "I don't agree that the status quo is acceptable. But just as I believe it would be wrong to have an immediate in-out referendum, so it would also be wrong to rule out any type of referendum for the future."